Answer:
Ten men's strength, ten men's length. Ten men can't break it, yet a young boy walks off with it.
Come up and let us go. Go down and here we stay.
What is that which, though black itself, enlightens the world without burning?
I am round as a bowl, deep as a tub, but all the world's water couldn't fill me up.
It can cause deflation of tires when dropped from the rear of spy cars.
You use it between your head and your toes, the more it works the thinner it grows.
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?